1. Field of the Invention
The invention concerns a device for measuring the interpupillary distance of a person, for example to assist with fitting eyeglass frames, comprising a box; a generally flat front panel of said box adapted to be applied to the face of a person in a known relative position so that a main axis normal to said front panel and a horizon plane containing said main axis respectively pass equidistantly between and through the centers of the eyes of said person which are at a fixed distance from said front panel; windows in said front panel adapted to be aligned with the eyes of said person; a convergent master lens in said box near said front panel with its optical axis coincident with said main axis; a point light source optically on said main axis at a position such that it determines for said person viewing it through said master lens a determined observation distance, light emitted by said point source forming respective corneal reflections on the corneas of said person; a measuring scale in said horizon plane normal to said main axis, optical images of said corneal reflections coinciding with respective divisions on said scale; means for identifying said respective divisions; and means for calculating and displaying an interpupillary distance corresponding to an identified pair of divisions.
It will be noted that the main axis of the device is normal to the general plane of the face of the person and that the horizon plane is referred to the attitude of the person with the head held straight and with the gaze focused on the horizon. Also, the front panel substantially corresponds to the normal position of the eyeglass lenses.
2. Description of the Prior Art
Patent document FR-A-1 506 352 describes an interpupillary distance measuring device which comprises the basic optical system, namely the convergent master lens and a point source optically on the main axis of the box to define an observation distance for the person and to form spots of light at the center of the corneas of the person. In this interpupillary distance measuring device the master lens is movable along the main optical axis from an origin position near the front panel at which the point source is optically at the focus of the master lens, which corresponds to an infinite observation distance. An eyepiece is formed in the box on the main axis opposite the front panel, optically coincident with the point source. Two alidades with a vertical line are disposed substantially in the plane of the front panel and are movable in the horizon plane. An observer viewing through the eyepiece aligns the vertical lines of the alidades with the respective corneal reflections. Graduated rules fastened to the alidades enable the interpupillary distance to be read off.
This interpupillary distance measuring device, the optical design of which is excellent, suffered from lack of precision in the maneuvering of the alidades and in the reading of the graduated rules. The patent document U.S. Pat. No. 4,591,246 describes an interpupillary distance measuring device comprising the same basic optical system in which the alidades are replaced by liquid crystal matrices at least the columns of which are addressable. A column that is turned on causes a vertical line to appear, the address of the column being representative of the abscissa of the line along the intersection of the plane of the front panel and the horizon plane. The observer moves the vertical line by causing the address generators to be incremented or decremented. A microprocessor controlling all of the electronic circuitry converts into the interpupillary distance the difference between the abscissas of the vertical lines, represented by the corresponding column addresses, allowing for the distance between the two matrices and their pitches. The interpupillary distance is displayed on a digital display device. It will be noted that if the liquid crystal matrices rows can also be addressed this interpupillary distance measuring device can be used to determine the position in a vertical plane of the bottom part of an eyeglass frame relative to the center of the cornea.
The accuracy and ease of use of this interpupillary distance measuring device are significantly improved as compared with the interpupillary distance measuring device of document FR-A-1 506 352; however, it still requires action by an observer to adjust the coincidence of the vertical lines on the measuring scale with the images of the corneal reflections, especially as the coincidence for the right eye and that for the left eye must be adjusted independently of each other. Apart from errors due to the individual skill of the observer, distractions may cause the person to shift their gaze from the image of the point source during the adjustment, jeopardizing the accuracy of the measurement.
It was therefore seen to be desirable to have available an interpupillary distance measuring device capable of determining the coincidence of the images of the corneal reflections with divisions on a measuring scale and of displaying the interpupillary distance without an observer having to take part in the measurement process.
The patent document U.S. Pat. No. 4,881,806 describes an automatic interpupillary distance measuring device in which there is formed for each eye an array of corneal reflections with parallel lines of light perpendicular to the horizon plane, with an associated position reference. A combination of lenses and point sources defines an observation distance for the subject. A rotating mirror is optically coincident with the point source, its rotation axis being perpendicular to the horizon plane. The images of the corneal reflections and the position references, as captured by the rotating mirror and appropriately focused, are scanned across a slit parallel to the axis of the mirror in a mask behind which is located a photo-electric sensor. Analyzing the signal supplied by the photo-electric detector makes it possible to locate the corneal reflections relative to the position references. It will be noted that, in practise, and for each eye, only the corneal reflection corresponding to the line of the array nearest the center of the cornea forms an image captured by the rotating mirror, given the divergent nature of the mirror formed by the cornea.
This interpupillary distance measuring device does not require any adjustment of cross-hairs or alidades by an observer and is therefore automatic or "impersonal", the latter expression meaning that it does not depend on the individual skill of an observer. However, it requires the use of a mirror rotating at a regular speed: also, the processing of the signals from the photo-electric sensor is relatively complex given that the sensed measurement reference is memorized in a first phase and the corneal reflections are located relative to the memorized reference in a second phase.
The patent document FR-A-2 620 927 describes apparatus for measuring the parameters needed to fit optical lenses to a frame. This apparatus essentially comprises an optical camera using charge transfer photosensitive elements, a telemetry device for recording the distance from the subject to the camera and a computer with associated software. An image of the face of the subject wearing the eyeglass frame is formed and the distance from the subject to the camera is recorded to define the scale of the image. The image signals are processed by the computer to extract from them the relevant parameters. The actual measurement process is in fact carried out by the software, no description of which is given.
An object of the invention is an interpupillary distance measuring device using for the most part the basic optical system from FR-A-1 506 352 and U.S. Pat. No. 4,591,246 but which is automatic and therefore impersonal, in the sense explained above and furthermore has no moving parts.